Reciprocating piston-based engines have provided satisfactory performance in a variety of applications. However, the design is not without shortcomings. Several of these limitations have motivated engineers to pursue engine designs that depart from the traditional reciprocating piston tradition.
First, the reciprocating piston engine can be relatively complex. The majority of economical models designed for automotive use have four or more pistons. Even lighter duty models, designated for low horsepower applications, still necessitate a number of valves, valve trains, valve cams, valve lifters, crankshaft and connecting rods, bearings, and the like. This duplication of power production elements (pistons), and related ancillary components, results in an increased probability of mechanical failure. In an eight cylinder embodiment for example, there are eight connecting rods that may crack, sixteen or more valves and connecting components that may fail, and other potential problematic occurrences exist.
Additionally the reciprocating piston engine often has a lower power to weight ratio than is needed or is provided by alternative designs. While some applications may be tolerant of additional engine mass, other applications may significantly benefit from lighter engines capable of producing comparable power. For example, race cars, All Terrain Vehicles (ATVs), snowmobiles, and the like become more agile with a reduced mass engine. Further, yard equipment and portable construction equipment benefit from the increased portability that accompanies a lighter engine.
The reciprocating engine design also suffers from inherent power inefficiencies. For example, at top dead center, significant power losses are experienced from the absence of any appreciable force vector being applied to the crankshaft. Additionally, most reciprocating designs require the exhaust valve to open when the power stroke is only partially completed. This results in lost potential energy being fully and effectively transferred to linear action of the piston. The requirement for an oil sump, and cooperating cooling means, can consume more than half of the engine's potential horsepower. Further, the relative difficulty involved in the starting of reciprocating engines necessitates keeping the engine running at idle when corresponding equipment (e.g., lawnmower, automobile) is temporarily stationary. This results in wasted fuel and unnecessary pollutant emissions.
The reciprocating piston engine is disadvantageously prone to catastrophic failure. Since reciprocating components endure extreme g-forces when abruptly changing from one linear direction to another (e.g., as a piston transitions from the compression stroke to the power stroke), the reciprocating piston engine is prone to catastrophic failure. A shattered connecting rod, broken timing chain, or other mechanical failure is capable of completely incapacitating the engine.
Other designs operate at diminished capacity when components fail. This feature is particularly beneficial in critical use applications, where advance notice of gradual demise (often referred to as Graceful Degradation) is preferred to instantaneous and complete failure of the engine. For example, it is desirable for a snowmobile to “limp” back to civilization, versus leaving its rider stranded in the wilderness.
Several rotary engine designs attempt to resolve the shortcomings of the reciprocating piston engine noted above, but many use excessively complex mechanisms or geometries to achieve their goals. For example, many use a variable rotary motion or “planetating” motion (e.g., the Wankel engine design) which often necessitates complex and non-uniform cylindrical piston bores. Therefore, there is a need for a lightweight, efficient, simple, and durable non-planetating (interchangeably referred to as “straight shaft”) rotary engine.